Dissolving pulp and a method for production thereof

ABSTRACT

A method of preparing dissolving pulp. The method includes physically separating a kraft pulp or a kraft hydrolysis pulp into first and second fractions, the first fraction having a relatively low lignin content and the second fraction having a relatively high lignin content.

FIELD

This invention relates to the treatment of kraft pulp in the productionof dissolving pulp.

BACKGROUND

Dissolving pulp is a raw material for manufacturing cellulosederivatives and regenerated cellulose. Production of dissolving pulp isgrowing. There are two commercial processes used for production ofdissolving pulp. One is the relatively well known acid sulfite process,the other being production of dissolving pulp from pulp produced via aprehydrolysis kraft process, which is relatively new. It is commonlyfound that adaptation of current kraft-based installations to dissolvingpulp production has not been entirely successful, at least to the extentthat it seems that dissolving pulps obtained to date have lowerreactivity than those obtained via sulfite-based processes.

Fock reactivity, defined as the % of cellulose dissolved under theconditions specified[1], is an important parameter in determining thesuitability of a dissolving pulp for such purposes. Reactivity is ameasure of a dissolving pulp's ability to chemically react with, forexample, carbon disulfide and sodium hydroxide in rayon production, ormore specifically the ability for hydroxyl groups on the glucose unitsof cellulose chains to react with carbon disulfide.

Production of pulps described as suitable for lyocell manufacture hasbeen described in the patent literature, for example, in U.S. PatentPublication No. 2009/0165969.

SUMMARY

In one aspect, the invention is a method of preparing dissolving pulp,the method comprising the steps of:

(i) separating a kraft pulp into a fiber fraction and a fines fraction;and(ii) processing the fiber fraction, to produce a dissolving pulp.

The separation step is carried out so as to obtain a first fractionhaving a relatively low lignin content and a second fraction having arelatively high lignin content. In certain embodiments, the separationstep includes separating pulp particles on the basis of size, whereinparticles of the first fraction are relatively large and particles ofthe second fraction are relatively small. The second fraction can have alignin content of at least about 15% by weight, or 16%, of 17%, or 18%,or 19%, or 20%, or 21%, or 22%, or 23%, or at least 24%. The firstfraction can have a lignin content of no more than about 1%, or 2%, or3%, or 4%, or 5%, or 6%, or 7%, or 8%, or 9%, or 10%, or 11%, or 12%, or13%, or no more than about 14%.

In one aspect, the kraft pulp is subject of separating step (i) is thusseparated into the fiber and fines fractions according to size toproduce a fines fraction having a lignin content higher than the lignincontent of the fiber fraction.

In embodiments, the unseparated kraft pulp has a kappa number of between10 and 50.

The kraft pulp subject of separating step (i) can have a kappa number ofbetween 10 and 40.

The kappa number of the first fraction can be no more than about 80% thekappa number of the unseparated pulp.

The method can be used with kraft pulp produced by a conventional kraftpulping process, or the kraft pulp can be a prehydrolysis kraft pulp. Inthe latter case, the kraft pulp would generally have a kappa number ofno more than about 20. The pulp may be produced as a separate stock, andprocessed separately e.g., in a separate mill, or kraft pulping woodchips to produce the kraft pulp may be carried out in-line along withsubsequent steps to produce the dissolving pulp. The wood chips can besubjected to steam/water prehydrolysis prior to the kraft pulping.

The method can thus include a step of, prior to step (i), kraft pulpingwood chips to produce the pulp. Wood chips can include softwood and thewood chips can be kraft pulped to obtain a pulp having a kappa number ofe.g., less than 40, or between 20 and 40, or between 25 and 40, orbetween 30 and 40. Softwood chips can include e.g., spruce, hemlock,fir, larch, or combinations of any of the foregoing. Hardwood chips canbe kraft pulped to obtain a pulp having a kappa number of less than 20,or between 10 and 20, or between 10 and 15. Hardwood chips can includee.g., maple, aspen, birch, or combinations of any of the foregoing.

According to certain embodiments, wood chips are subjected to steamand/or water prehydrolysis prior to kraft pulping. The kraft pulp can beobtained from a conventional kraft process without a prehydrolysis step,and the pulp subject of separating step (i) can have a kappa number ofbetween 10 and 40, or between 30 and 40.

In embodiments, the lignin content (wt %) of the second fraction is atleast 1.1 the lignin content of the first fraction, more preferably atleast 1.2, 1.3, 1.4 or 1.5 of the first fraction. Further, between 85and 95 wt % of the unfractionated kraft pulp can be contained in thefirst fraction obtained in step (i).

The method can further include a step (iii), of exposing the fiberfraction to a cellulase, xylanase, and/or mannase enzyme to decrease theintrinsic viscosity of the pulp fraction by between 50 and 700 ml/g(0.5% cellulose in a cupriethylenediamine (CED) solution). Morepreferably, the decrease is in the range of from 100 to 300 ml/g. Thecellulase enzyme can be a multicomponent enzyme or an enzyme having asingle type of catalytic activity. FiberCare D™ can be used in a dosage0.1 to 10 u/g of dry pulp, preferably, 0.3 to 2 u/g of pulp. Examples ofother commercially available enzymes are those sold under the namesFiberCare U, FiberCare R, Celluclast, Fiberzyme CS, Fiberzyme G200,Fiberzyme LBR, Optimase Cx, Pyrolase HT cellulose, Pulpzyme HB, PulpzymeHC, Luminase PB-100 and Mannaway®. These can be used alone or incombination with each other.

The method can include exposing the pulp fraction to one or morecationic polymers during such an enzyme treatment step. Cationicpolymers can be included in enzymatic steps, particularlycellulase-catalyzed steps to enhance enzymatic performance. The polymercan be, but are not limited to a cationic polyacrylamide (CPAM),polydiallyldimethylammonium chloride (DADMAC), polyethylenimine (PEI),polyaluminum chloride (PAC), cationic starch, or a dual polymer system,and the cationic polymer(s) can further include an anionic polymer,microparticle-containing system, and/or a silica-based cationicpolymers. If present, polymer is present at e.g., a dosage in the rangeof 0.01 to 1000 ppm.

In embodiment the method includes a step of (iv) bleaching the firstpulp fraction before, after or as part of step (iii). The method canfurther include a step of (v) exposing the bleached pulp obtained instep (iv) to a xylanase. Such method can further include the step of(vi) subjecting the pulp obtained in step (v) to alkali.

Step (vi) can be a hot alkali treatment, or step (vi) can be a coldalkali treatment with the caustic soda concentration in the range of 1to 12 wt %, preferably, 5 to 12 wt %, even more preferably 8 to 10 wt %.

In embodiments that include enzymatic treatment step (iii), the methodcan further include recovering active enzyme of step (iii) and cyclingthe recovered enzyme back into step (iii). The amount of enzyme recycledcan be in the range of 30 to 90%, more preferably 50 to 80%. Inembodiments that include step (v), the method can include recoveringactive enzyme of step (v) and cycling the recovered enzyme back intostep (v).

The dissolving pulp produced typically has a relatively low lignincontent of no more than about 0.3 wt %, no more than about 0.2 wt %, orno more than about 0.1 wt %.

The dissolving pulp produced typically has an α-cellulose content of atleast about 88 wt %, or at least about 90 wt %, or at least about 92 wt%, or at least about 94 wt %, or at least about 96%, or higher.

The dissolving pulp produced typically has a pentosan content of no morethan about 8 wt %, or less than about 7 wt %, or less than about 6 wt %,or less than about 5 wt %, or less than about 4 wt %.

The pulp of step (i) can be unbleached pulp.

The separating step is carried out by e.g., centrifugal separation, suchas by use of a hydrocylone or series of screens, or similar setups, orit can be by screening of the kraft pulp into the fiber fraction andfines fraction.

The method can include (iv) bleaching the pulp, before, after or as partof step (iii). The method can further include the step of (v) exposingthe bleached pulp to a xylanase. The method can also include the step of(vi) subjecting the pulp produced in step (v) to alkali. Step (vi) canbe a hot alkali treatment or a cold alkali treatment, as discussedfurther below.

The method can also include a step of refining the fiber fraction toproduce the dissolving pulp. The Fock reactivity of the dissolving pulpproduced can be at least 20% more preferably, 30%, 40%, 50%, morepreferably at least 60%.

The method can also include a step of (a) exposing the second i.e. finesfraction to a cellulase enzyme, mechanical and/or alkali treatment.

The method can further include a step of (b) bleaching the finesfraction i.e., step (a) can be carried out prior to, subsequent to, orsimultaneously with step (b). Such steps can be carried out in differentorders, and can be repeated.

It can be possible to process the fines fraction separately to produce apulp that might be used as stock to produce a paper product, or thefines could be, or at least a portion of the fines could be, processedinto a dissolving pulp and rejoined with the fiber fraction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow diagram incorporating process steps of the presentinvention; and

FIG. 2 shows Fock reactivity and viscosity of enzyme treated dissolvingpulps produced from the prehydrolysis kraft-based process from a mixedof hardwood. Enzymatic treatment was performed at pH 5.0 and temperature55° C. for 2 hours.

FIG. 3 shows the effect of the addition of 250 ppm CPAM on the change inviscosity over time of pulp (consistency 3%, pH 4.8 and 55° C.) treatedwith cellulase (mg/g pulp): 0.5 (▪), 1 (), and 1.5 (▴). Cellulasecharge (mg/g pulp) with no added CPAM: 0.5 (□), 1 (◯) and 2 (hollowinverted triangles).

FIG. 4 shows Fock reactivity of pulp (3% pulp consistency, pH 4.8 and55° C.) after 12 hours (hatched bars) and 24 hours (unhatched bars) ofcellulase treatment. The first, third and fifth pairs of bars are 0.5, 1and 2 mg of cellulase charge per gm of pulp with no added CPAM. Thesecond, fourth and sixth pairs of bars are 0.5, 1 and 1.5 mg ofcellulase charge per gm of pulp and 250 ppm CPAM.

DETAILED DESCRIPTION

This invention discloses a cost effective and environmentally friendlymethod for improving the product quality parameters and processingefficiency in the production of dissolving pulp by the prehydrolysiskraft process or kraft process.

Pulp quality, such as Fock reactivity, purity e.g., high alpha-celluloseand low pentosan content, accessibility, is important for dissolvinggrade pulp, these quality parameters being important in the downstreamproduction process regardless of the pathways of functionalizingcellulose during viscose production and processing.

Dissolving pulp from a prehydrolysis kraft pulping process or from thekraft pulping process contains undesirable carbohydrates components,including residual hemicelluloses after cooking, such as glucomannan,glucuronoxylan. In addition, some degraded cellulose chains might be tooshort or altered to suit the properties of certain products such as cordrayon, high tenacity staple fiber, modal fiber, and should therefore beremoved. Portions of the pulp that are smaller in size, such as raycells and fines, have inferior properties compared to those that arelarger in size, such as tracheid.

According to the present invention, the dissolving pulp quality e.g,purity of the alpha-cellulose, reduced pentosan content, increasedreactivity and accessibility, can be improved cost-effectively byadditional purification steps or combining the purification stage/stageswith the existing manufacturing process, especially in brown stockoperations and bleach plant operations. The purification of dissolvingpulp can be conducted in a controllable manner to meet the end-usespecifications.

As shown in FIG. 1, a fractionation stage in which a rejects portioncontaining relatively large proportions of fines and ray cells isseparated from fibers after prehydrolysis and kraft pulping can includeadditional steps such as subsequent bleaching, enzyme treatment and/oralkali purification.

Fractionation

U.S. Pat. No. 4,731,160 describes the fractionation method formechanical pulp into a major fraction and fines fraction. U.S. Pat. No.7,005,034 discloses a method for production of mechanical pulp, whereinthe pulp is fractionated after a first refinery stage for separating thefine material from the pulp to facilitate the refining of the fibers inthe second stage. U.S. Patent Publication No. 2007/0023329 describes amethod for selective removal of ray cells from pulp by combiningscreening and centrifugal cleaning.

Unknown to the inventors is the description in literature of thefractionation of a pulp to be used as, or in the production of, adissolving pulp, either from the prehydrolysis kraft pulping process orthe kraft process in order to obtain a fraction containing a loweredlignin content.

According to the present invention, unbleached pulp from the kraftpulping stage contains a small percentage of ray cells and fragments offibers and middle lamellae. This portion of pulp is called fines. Thefines portion has significant higher content of impurities such aslignin, resin, extractives, and non-process elements like iron, calcium,and magnesium. In addition, the fines portion has a much slower drainagerate, and inferior bleach-ability resulting in higher bleach chemicalsconsumption and accessibility. Although the fines account for only asmall amount (usually less than 8%) of the mass of the whole pulp, theirnegative impact on the performance and efficiency of the bleaching,enzymatic treatment and alkali purification stages, as well as theoverall quality of the final dissolving pulp, is significant.

According to the invention, unbleached pulp is separated byfractionation into two fractions: a fiber fraction and a fines fraction,which are separately processed. This leads to overall chemical andenergy efficiencies and/or improved quality parameters of dissolvinggrade pulp ultimately obtained from the fiber fraction. FIG. 1illustrates an embodiment of the invention.

Dissolving pulp can be produced in a more cost-effective manner byfractionating pulp first to improve the chemical response and drainagecharacteristics of the majority of the dissolving grade pulp (the fiberfraction) in the bleaching, enzymatic treatment and/or alkalipurification step and/or mechanical refining of fibers. The finesfraction separated by the fractionation step, can be processed bybleaching, alkali extraction and enzymatic treatment.

It thus becomes possible for regular kraft production to be the basisfor production of dissolving grade pulp suitable for viscose process(xanthation), or for other cellulose derivatives production.

The process can be varied based on the desired purity of pulp.

Unbleached pulp from the kraft pulping stage is separated into a minorfines fraction and major fiber fraction. The separation may beaccomplished using a screen, or centrifugal separators (cyclones). Themajor fiber fraction is typically subjected to enzymatic treatment andthen subsequent bleaching sequences to produce fully bleached pulphaving higher brightness and whiteness than pulp bleached without aseparation stage at the same chemical dosage, or to produce dissolvingpulp at lower chemical dosage and lower energy consumption for givenpulp brightness and whiteness. After bleaching, the fiber fraction issubjected to enzymatic treatment and/or cold alkali purification and/orrefining to increase the α-cellulose content to the specific target, forexample, about 98%, and increase Fock reactivity, in a more efficientway in terms of energy and chemical consumption when compared to asimilar process that does not include such a fractionation stage. Table1 shows the mass percentage of lignin content for different pulpfractions of one kraft brown stock pulp.

TABLE 1 Fiber Fraction (mesh size) % of total (wt %) Lignin (wt %)  >5089.20 5.50  50-100 4.00 7.60 100-200 1.07 11.40 200-300 3.32 13.00 <3002.24 24.30

As shown in Table 1, the lignin content is much different in differentfractions of the brown stock pulp, the longest fraction (>50 mesh size)accounts for about 90% of the mass, but the lignin content is the lowest(5.5%). One can thus see that (i) residual lignin in unbleached pulp isunevenly distributed; and (ii) the smallest fine fraction (mostly raycells) contains much higher lignin content than the fiber tracheids(long fibers). If the fines fractions would be separated from thetracheids, due to the significantly lower lignin content, smalleramounts of bleaching chemicals would be needed in the subsequent bleachplant. Also, if the fines are removed, the filterability of thedissolving pulp should be improved.

As the separation processes described herein are, ultimately, toward theproduction of a dissolving pulp, the pulp subject of the separation,fractionation or screening process has a suitably low overall lignincontent. This is reflected in the kappa number of the pulp prior to theseparation step. For conventional softwood kraft pulp, the kappa numbermay be in a range of 30 to 40; for a prehydrolysis kraft-baseddissolving pulp, in particularly, hardwood pulp, the kappa number can besubstantially lower, in the range of 10 to 16, as lignin is generallyconsidered an undesirable component of dissolving pulps. The resultsshown in Table 1 were obtained using a conventional softwood kraft pulphaving a kappa number of about 38.

The fines fraction can be treated separately, and also more efficientlyby the processes involved, since the volume of the fraction is smallerthan that of the long fiber fraction. As shown in Table 1, the masspercentage of the fine fraction (passed 200 mesh) is about 5%, but itslignin content is several times that of the fiber fraction (fibersretained on 100 mesh or longer). Table 2 shows lignin and metal contentof pulp fibers and ray cells, the pulp being without any fractionation,and the ray sells being obtained by screening. As can be seen, the raycells, a major component of the fine fraction, have a much higher ironand other metal content than the pulp as a whole, so fines separation byfractionation would decrease iron content of the pulp.

The fines fraction can thus be subjected to an acid treatment stage toremove the iron more efficiently, and then bleached by a bleachingsequence (e.g. D₀E_(OP)HE_(P)D₂: D₀ chlorine dioxide pre-bleaching,E_(OP): oxygen and peroxide re-enforced alkali extraction, H:hypochlorite, E_(P): peroxide re-enforced extraction, D₂: secondchlorine dioxide brightening stage). The fines fraction may also betreated by the enzyme treatment and alkali purification if needed, againin a more efficient way.

In tests it was found that removal of the fines leads to the dissolvingpulp having improved drainage properties. This is shown in Table 3 bythe increase of the filterability (or CSF freeness) of the pulp that was60% maple, 30% aspen and 10% birch, all of which are hardwoods. Inaddition, as shown in Table 3, the brightness is much higher for thelong fiber fraction than the shorter fiber fraction, while S₁₈extractives, and pentosan for the long fiber fraction, are lower.

TABLE 2 Lignin Mn Cu Fe Mg Ca [%] [mg/L] [mg/L] [mg/L] [mg/L] [mg/L]Pulp 3.38 100 1.2 12 270 45 Ray Cell 8.1 178 25 146 587 123

TABLE 3 Long fibers Parameter Unit Feed (>100 mesh) Short fibers (<200mesh) S₁₀ % 6.5 5.6 N/A S₁₈ % 4.2 3.56 4.89 Extractives % 0.15 0.10 0.41Ash % 0.04 0.02 Pentosan % 2.88 2.8 2.87 Brightness % 89 90.5 82.5 ISOFilterability 301 428 17

Enzymatic Treatment

In a prehydrolysis kraft process, as in the example of pulp obtainedhere from the AV Nackawic mill, the majority of the hemicelluloses inwood chips is degraded in the acidic prehydrolysis step and dissolved inthe subsequent kraft cooking stage. Typically, unbleached pulp from thecooking process is bleached to about 90% ISO brightness by a five-stagebleaching sequence (for example, D₀E_(OP)HE_(P)D₂, D₀ chlorine dioxidepre-bleaching, E_(OP): oxygen and peroxide re-enforced alkaliextraction, H: hypochlorite, E_(P): peroxide re-enforced extraction, D₂:second chlorine dioxide brightening stage). Most of the lignin and woodextractives are also removed in the cooking and bleaching processes.Upon drying, the remaining cellulose fibrils are able to bind moreclosely to each other to form a compact structure. The inter fibrillarspaces of the pulp fibers collapse and thus the surface area and porevolume decrease [2-5]. This irreversible process is called hornificationwhich causes decreased pulp accessibility/reactivity [6,7]. The methodof drying influences the size of the fibril aggregate. Harsher drying(fast drying) produces larger lateral fibril aggregates, resulting inlower reactivity [8]. The hornification effect is more pronounced in thepresence of hemicellulose that has a very high bonding capability. Thisis one of the reasons why the hemicellulose content of dissolving pulphas to be reduced to a very low level.

Enzyme hydrolysis/degradation and chemical modification have beenproposed to increase the accessibility/reactivity of dissolving pulp[5,9]. Within the past few years, a monocomponent endoglucanase has beenused to treat dissolving pulp to increase accessibility/reactivity withpromising results [10,11]. Pure monocomponent cellulases of theendoglucanase type are commercially available.

The activation mechanism of dissolving pulp by endoglucanases is notcompletely understood, although hypotheses have been put forward[12,13]. It is generally believed that the attack on the less orderedcellulose regions by the endoglucanase leads to fiber wall swelling andthus an increase in accessibility towards solvents and reactants.Endoglucanase preferably degrades amorphous cellulose located on thefiber surface and between the microfibrils, which leads to increasedcrystalline surface exposure and to increased swelling ability andreactivity of the pulp [14]. Endoglucanase may also increase reactivityby attacking cellulose II [15,16]. Recent studies by Ibarra et al.reveal that the activation effect of endoglucanase is affected by themodular structure of the enzyme and the drying history of dissolvingpulp [17,18]. They found that endoglucanase with an inverting catalyticdomain and a cellulose binding domain is most effective in activatingcellulose, in particular when it is applied on never-dried pulps.

Most studies, however, have focused on sulfite dissolving pulp, andlittle has been reported in the literature on the effect of enzymetreatment on the accessibility/reactivity of the dissolving pulp from aprehydrolysis kraft process. The action of the endoglucanase on kraftpulp differs from that on sulfite dissolving pulp. For example, thereactivity of a sulfite dissolving pulp increased rapidly from about 70%to almost 100% within 10 min of incubation [9]. In contrast, postendoglucanase treatment of a kraft pulp increased its reactivity onlymarginally (from 19.1% to 22.9%) [19].

Endoglucanase may act on kraft pulp by a different mechanism, or it maybe that sulfite and kraft pulps differ in chemical and physicalproperties. Modification of the bleaching process of prehydrolysis kraftpulp production may improve its Fock reactivity, as well as its responseto enzyme treatment for reactivity improvement. In a recent study, aspruce prehydrolyzed kraft pulp was bleached with chlorite to remove theremaining lignin, in order to study the effect of residual lignin on theFock reactivity [20]. The results showed that the Fock reactivity of thepulp increased with decreasing kappa number (decreasing residual lignincontent), and the highest reactivity was obtained after complete ligninremoval using chlorite delignification. It was also found that thecarbohydrate composition had little influence on the pulp reactivity,but lower intrinsic viscosity either obtained by prolonged cooking orchlorite delignification correlated with higher pulp reactivity.

It has been found here that treating the pulp with xylanases and/orcellulases can significantly improve the purity and Fock reactivity ofthe dissolving pulp. Xylanases can have an effect on pore structure aswell as pentosan content. The pre-bleach application is the standardapplication point but xylanase can be effectively applied at any pointof the bleaching process where conditions are acceptable. Hemicellulase(such as xylanase) is an effective means to increase brightness andremove residual pentosan after the bleaching.

Accessibility to the substrate can limit the effectiveness of enzymetreatments, and especially so for dissolving grade pulps. For thisreason, it is most effective to have complementary activities workingsimultaneously. Cellulases and hemicellulases can complement each otherand achieve greater results than either alone. It is not always feasibleto remove most of a single component without disrupting other substratestructures. Multiple applications of a particular treatment can be moreeffective than a single treatment at the same net dose. Pentosan removalis more efficient when a series of enzyme treatments are carried out atdifferent points in the process, especially when there is a wash step ora different reactive step in between. This leads to the prospect of abrown stock treatment and a post bleach treatment with hemicellulases.

According to the present invention, the Fock reactivity of dissolvingpulp can increase from the enzyme treatment, as shown in FIG. 2 (thereactivity and viscosity of resulting dissolving pulp as a function ofthe cellulase (a multi-component cellulase sample, FiberCare D, fromNovozymes) dosage. It is evident that the reactivity increased from48.0% to 93.5% when increasing the cellulase dosage from 0 to 2 u·g⁻¹dry pulp, and the viscosity decreased from 665.8 ml·g⁻¹ to 354.7 ml·g⁻¹;a further increase in the cellulase dosage only resulted in a slightincrease in the Fock reactivity.

Other enzymes, such as FiberCare U, FiberCare R, Celluclast, FiberzymeCS, Fiberzyme G200, Fiberzyme LBR, Optimase Cx, Pyrolase HT cellulose,may also be used for this purpose.

As shown in FIG. 2, the decrease in the pulp viscosity correspondsroughly with the observed increase in the reactivity of dissolving pulp.

One or more cationic polymers, such as CPAM (cationic polyacrylamide)can be included in the cellulase treatment solution to enhance theperformance of the cellulase treatment step. FIG. 3 shows the effect onviscosity of the inclusion of CPAM at a concentration of 250 ppm in thecellulase enzymatic treatment of dissolving pulp for different enzymeconcentrations. It can be seen that the decrease in intrinsic viscosityover time is greater in the presence of CPAM for a given charge ofenzyme. The viscosity of cellulase charge of 1 mg/g from the CPAMaddition was lower than that of 2 mg/g cellulase charge without CPAM,for example, demonstrating that the cellulase efficiency wassignificantly improved due to the addition of CPAM.

The use of CPAM in the cellulase treatment can also enhance the increasein Fock reactivity, as shown in FIG. 4. As evident from the resultsshown, treating samples with cellulase-CPAM compositions resulted inpulp compositions have significantly higher Fock reactivity.

According to present invention, the pentosan content of the pulp can bedecreased significantly with the enzyme treatment, Ecopulp TX-800 (anindustrial enzyme product) as shown in Table 4. The yield losses forpulp treated by enzyme are from 1 to 1.3% per 1% increase in α-cellulosecontent.

Other enzymes may also be used, including Pulpzyme HB, Pulpzyme HC,Luminase PB-100, and Mannaway®.

TABLE 4 Addition Retention Enzyme (L/T) pH Consistency Temp. Time WashedK# Pentosan Ecopulp 0 7.4 14.23% 60° C. 30 min Yes*** 5.6 3.56 TX-800 A*Ecopulp 0.04** 7.4 14.23% 60° C. 30 min Yes*** 5.5 3.48 TX-800 A*Ecopulp 0.08** 7.4 14.23% 60° C. 30 min Yes*** 5.4 3.04 TX-800 A*Ecopulp 0.10** 7.4 14.23% 60° C. 30 min Yes*** 5.2 2.2 TX-800 A* 60° C.30 min Yes 60° C. 30 min Yes 60° C. 30 min Yes Samples were tested 24days after treatment. *Diluted 1 ml/1000 ml. **Additions - 0.4 ml/10 g,0.8 ml/10 g, 1.0 ml/10 g of diluted enzyme. ***Washed in sheet formerwith 15 L of water purified by reverse osmosis; dried in oven. BOD Usedthe 4L/T sample 1 ml enzyme diluted to 1000 ml 0.4 ml of the enzymediluted to 2000 ml: 1/1000 × 0.4/20000 = 0.0000002 ml of enzyme thatsample gave 12 mg/l BOD.

Alkali Treatment

Alkaline treatment/purification of pulp can principally be carried outin two ways, cold and hot alkaline purification:

-   a) Cold purification consisting of the treatment of pulp in    concentrated lye around room temperature, permitting short chain    material and microfibril fragments to dissolve. Cold purification    thus primarily involves physical changes to the pulp, and only a    small amount of alkali is consumed; and-   b) Hot alkali purification, performed at a higher temperature    (usually higher than 70° C., and in some cases, higher than 100° C.)    with relatively a lower alkali dosage (0.2-4 wt % on pulp). The more    accessible parts of the fiber react under these conditions, with the    formation of organic acids.

Cold Alkali Purification

The cold alkali treatment is a selective process of increasing the alphacellulose content of the dissolving pulp, decreasing the pentosancontent and increasing the pulp viscosity of dissolving pulp from theprehydrolysis kraft pulping process. Degraded cellulose can have a goodsolubility at about a 10 wt % NaOH concentration at room temperature,while the solubility of hemicelluloses increases with increasing NaOHconcentration.

Shown in Table 5 are the results obtained using a cold alkalipurification process. As can be seen, after cold alkali purificationunder the conditions specified, the pulp viscosity increased from 605 to641.6 ml/g, the α-cellulose content increased from 94.3 to 97.9% whilethe pentosan content decreased from 4.42 to 1.41%. Reactivity decreasedfrom 39.2% to 20.8%. The decrease reactivity is due to the increasedcrystallinity of the treated fibers.

TABLE 5 Reactivity Viscosity Pentosan Sample ID (%) (ml/g) (%)α-Cellulose (%) Before alkaline 39.2 605.0 4.42 94.3 treatment Afteralkaline 20.8 641.6 1.41 97.9 treatment Conditions: 9 wt % NaOHconcentration, 9 wt % pulp consistency, 30 minutes, 35° C. Each samplewas washed with deionized water until pH 6-7 then air dried.

Suitable ranges for the temperature and alkali concentration for thecold alkali purification are 5 to 40° C. and 5 to 18%. Under theseconditions, mainly physical changes occur. 35° C. is a preferredtemperature for the prehydrolysis kraft dissolving pulp. Cold alkalitreated pulp is sensitive to oxidative attack, which may lead to adecrease in the pulp viscosity. Reaction time can be short and theswelling reaction takes place almost instantaneously. Pulp consistencycan vary from 3.5 to 20%. The pentosan content of the pulp decreases asthe α-cellulose content is increased after the cold alkali treatment.

Hot Alkali Purification

In contrast to cold purification, the hot alkali treatment involveschemical reactions in the purification process. Swelling is limited, asthe concentration of lye in the reaction mixture is only 0.2-4 wt %NaOH. The degree of purification is regulated by the alkaliconcentration, as well as by the temperature, time and pulp consistency.In comparison to the cold alkali treatment, the main drawbacks of thehot alkali method are the consumption of steam for heating the pulp andhigher yield loss of pulp. A yield loss of about 3% is expected per 1%increase in α-cellulose content.

Mechanical Treatment

Mechanical treatment of pulp fibers, such as grinding, refining, canimprove the dissolving pulp reactivity. Fock reactivity was determinedat 18° C., by a dissolving pulp sample ground in a coffee grinder, i.e.,for refining/grinding. As indicated by the results shown in Table 6,Fock reactivity increases with grinding time. Fiber length wasdetermined by FQA.

TABLE 6 No. Grinding Time Fiber Length (length weighted) Fock Reactivity1 0 min 0.545 mm 55.90% 2 0.5 min   0.536 mm 59.10% 3 1 min 0.514 mm61.08% 4 3 min 0.486 mm 65.56% 5 5 min 0.466 mm 68.67% 6 10 min  0.433mm 75.13%

Enzyme Recycling

Enzyme recycling relates to the process economics of the enzymetreatment. After use, the filtrate containing active enzymes and can berecycled/reused. Table 7 shows effects of recycle of filtrate fromenzymatic treatment on dissolving pulp reactivity.

TABLE 7 No Addition Addition Addition Fresh additional of 20% of 35% of50% enzyme fresh fresh fresh fresh Sample Control treatment enzymeenzyme enzyme enzyme Reactivity/% 47.67 94.42 84.59 86.45 89.19 90.99Enzymatic treatment conditions were: pulp weight 10 g o.d.; pulpconsistency, 4%; temperature 55° C.; pH 5; and time, 2 hours.

The enzyme dosage of the fresh enzyme treatment was 1.0 u/g o.d. pulp.

As used herein, the terms, “comprises” and “comprising” are to beconstrued as being inclusive and open ended, and not exclusive.Specifically, when used in this specification including claims, theterms, “comprises” and “comprising” and variations thereof mean thespecified features, steps or components are included. These terms arenot to be interpreted to exclude the presence of other features, stepsor components. As used herein, the terms “about”, and “approximately”when used in conjunction with ranges of dimensions, concentrations,temperatures or other physical or chemical properties or characteristicsis meant to cover slight variations that may exist in the upper andlower limits of the ranges of properties/characteristics.

The contents of all documents mentioned herein are incorporated hereinby reference as though reproduced in their entirety.

REFERENCES

-   1. Tian, C., Improvement in the Fock test for determining the    reactivity of dissolving pulp. Tappi Journal 2013; 12(11):21-26.-   2. H. A. Krässig, Das Papier 38 (1984)571-582.-   3. T. N. Kleinert, Holzforschung 29 (1975) 134-135.-   4. T. Oksanen, J. Buchert, L. Viikari, Holzforschung 51 (1997)    355-360.-   5. K. D. Sears, J. F. Hinck, C. G. Sewell, J. Appl. Polym. Sci.    27 (1982) 4599-4610.-   6. N. M. S. El-Din, F. F. A. El-Megeid, Holzforschung 48 (1994)    496-500.-   7. G. Jayme, U. Schenck, Das Papier 3 (1949) 469-476.-   8. Chunilall, V, Bush, T. Larsson, P T. Iversen, T. Kindness, A. A    CP/MAS (13)C-NMR study of cellulose fibril aggregation in eucalyptus    dissolving pulps during drying and the correlation between aggregate    dimensions and chemical reactivity. HOLZFORSCHUNG 6(6): 693-698    (2010).-   9. H. A. Krässig, Methods of activation. In: H. A. Krässig (ed)    Cellulose: Structure, accessibility and reactivity, 1st edn. Gordon    and Breach Science Publishers, Amsterdam, The Netherlands, pp    215-276 (1993).-   10. Ann-Charlott Engström, Monica Ek,* and Gunnar Henriksson.    Improved Accessibility and Reactivity of Dissolving Pulp for the    Viscose Process: Pretreatment with Monocomponent Endoglucanase.    Biomacromolecules 2006, 7, 2027-2031.-   11. V. Gehmayr, G. Schild, H. Sixta, Cellulose 18 (2011) 479-491.-   12. Henriksson G, Christiernin M, Agnemo R. Monocomponent    endoglucanase treatment increases the reactivity of softwood    sulphite dissolving pulp. Journal of Industrial Microbiology and    Biotechnology 2005; 32:211-4.-   13. Rabinovich M S, Melnik M S, Bolobova A V. The structure and    mechanism of action of cellulose enzymes. Biochemistry 2002;    67:850-71.-   14. G. Henriksson, M. Christiernin, R. Agnemo, J. Ind. Microbiol.    Biotechnol. 32 (2005) 211-214.-   15. L. Rahkamo, L. Viikari, J. Buchert, T. Paakkari, T. Suortti,    Cellul. 5 (1998) 79-88.-   16. R. H. Atalla, Conformational effects in the hydrolysis of    cellulose. In: R. D. Brown Jr., L. Jurasek (ed) Hydrolysis of    cellulose: Mechanisms of enzymatic and acid catalysis, 1st edn.    American Chemical Society, Washington D.C., pp 55-69 (1979).-   17. Ibarra, D., Kopcke, V., and Ek, M., 2010. Behavior of different    monocomponent endoglucanases on the accessibility and reactivity of    dissolving-grade pulps for viscose process. Enzyme and Microbial    Technology, 47(7): 355-362.-   18. Ibarra, D., Kopcke, V., Larsson, P. T., Jaaskelainen, A. S., and    Ek, M., 2010. Combination of alkaline and enzymatic treatments as a    process for upgrading sisal paper-grade pulp to dissolving-grade    pulp. Bioresource Technology, 101(19): 7416-7423.-   19. Verena Gehmayr1 and Herbert Sixta. Dissolving pulps from enzyme    treated kraft pulps for viscose application. Lenzinger Berichte    89 (2011) 152-160.-   20. Javed, M A, Germgard, U. The reactivity of prehydrolyzed    softwood kraft pulps after prolonged cooking followed by chlorite    delignification. BIORESOURCES, 6(3): 2581-2591 (2011).

1. A method of preparing dissolving pulp, the method comprising thesteps of: (i) physically separating a kraft pulp into first and secondfractions, the first fraction having a relatively low lignin content andthe second fraction having a relatively high lignin content; and (ii)processing the first fraction to produce a dissolving pulp.
 2. Themethod of claim 1, wherein step (i) includes separating pulp particleson the basis of size, wherein particles of the first fraction arerelatively large and particles of the second fraction are relativelysmall.
 3. The method of claim 2, wherein the second fraction has alignin content of at least about 15% by weight.
 4. The method of claim3, wherein the first fraction has a lignin content of no more than about6% by weight.
 5. The method of claim 4, wherein the unseparated kraftpulp has a kappa number of between 10 and
 50. 6. The method of claim 5,wherein the kappa number of the first fraction is no more than about 80%the kappa number of the unseparated pulp.
 7. The method of claim 6,further comprising the step of, prior to step (i), kraft pulping woodchips to produce the pulp.
 8. The method of claim 7, wherein said woodchips are subjected to steam and/or water prehydrolysis prior to saidkraft pulping.
 9. The method of claim 7, wherein the kraft pulp isobtained from a conventional kraft process without a prehydrolysis step,and the pulp subject of separating step (i) has a kappa number ofbetween 10 and
 40. 10. The method of claim 9, wherein the lignin content(wt %) of the second fraction is at least 1.1 the lignin content of thefirst fraction.
 11. The method of claim 10, wherein between 85 and 95 wt% of the unfractionated kraft pulp is contained in the first fractionobtained in step (i).
 12. The method of claim 11, further comprising thestep of (iii) exposing the first pulp fraction to a cellulase, xylanase,and/or mannase enzyme to decrease the intrinsic viscosity of the pulpfraction by between 50 and 700 ml/g (0.5% cellulose in acupriethylenediamine (CED) solution), optionally exposing the pulpfraction to one or more cationic polymers wherein said polymer can beselected from the group consisting of cationic polyacrylamide (CPAM),polydiallyldimethylammonium chloride (DADMAC), polyethylenimine (PEI),polyaluminum chloride (PAC), cationic starch, a dual polymer system,wherein the cationic polymer can further comprise an anionic polymer,microparticle-containing system, and/or a silica-based cationicpolymers.
 13. The method of claim 12, wherein the enzyme comprisesFiberCare D™, present in a dosage 0.1 to 10 u/g of dry pulp; furthercomprising the step of (iv) bleaching the first pulp fraction before,after or as part of step (iii); further comprising the step of (v)exposing the bleached pulp obtained in step (iv) to a xylanase; furthercomprising the step of (vi) subjecting the pulp obtained in step (v) toalkali; and wherein step (vi) comprises a hot alkali treatment, or step(vi) comprises a cold alkali treatment and the caustic sodaconcentration is in the range of 1 to 12 wt %.
 14. The method of claim13, further comprising recovering active enzyme of step (iii) andcycling the recovered enzyme back into step (iii), wherein amount ofenzyme recycled is in the range of 30 to 90%.
 15. The method of claim 6,wherein step (i) comprises centrifugal separation.
 16. The method ofclaim 6, wherein step (i) comprises screening of the kraft pulp into thefirst and second fractions.
 17. The method of claim 6, wherein the kraftpulp of step (i) is a prehydrolysis kraft pulp.
 18. The method of claim6, further comprising the step of refining the fiber fraction to producethe dissolving pulp.
 19. The method of claim 18, wherein the dissolvingpulp produced has a lignin content of no more than about 0.3 wt %. 20.The method of claim 19, wherein the dissolving pulp produced has anα-cellulose content of at least about 88 wt %.
 21. The method of claim20, wherein the dissolving pulp produced has a pentosan content of nomore than about 8 wt %.
 22. The method of claim 21, wherein the Fockreactivity of the dissolving pulp produced is at least 50%.